1. Field of the Invention
The present invention relates to a portable illumination apparatus which has the high brightness and realizes uniform illumination, and an image projection apparatus using the illumination apparatus.
2. Description of the Related Art
As a light source in an illumination apparatus for use in an image projection apparatus such as a video projector, a discharge type bulb lamp light source is generally used in combination with a parabolic reflector or an ellipsoid reflector. However, in the bulb lamp light source, since a lamp bulb itself or an electrode forms a shadow, an unevenness of radiation distribution is generated and there occurs a so-called “reduced dark spot”. Further, it has radiation angle characteristics of a relatively narrow (non-flat) distribution. Therefore, in such a discharge type lamp, when Koehler illumination is carried out, the “reduced dark spot” is generated at the center, or shading at the circumference becomes prominent, which leads to lack of illuminant uniformity. Thus, an integrator such as a fly-eye lens must be usually employed, which results in a large-scale illumination apparatus.
On the contrary, a light emitting diode (which will be referred to as an LED hereinafter) has advantages such as a long duration of life, a single color luminescence, a high color reproducibility, as compared with the conventional bulb lamp light source. Furthermore, in recent years, its luminous efficiency is getting close to that of the bulb lamp, and it is expected that the LED will exceed the bulb lamp in this regard. Thus, various kinds of image projection apparatuses such as a video projector using such a LED as a light source have been proposed.
Since this LED is a so-called diffusion (Lambertian) light source, when the LED is used as a light source in an illumination apparatus, how to efficiently condense the light and lead it to an illumination area is difficult, which must be contrived. Moreover, an LED chip is constituted by bonding an electrode to an electrode structure and an element.
Since the LED has such a chip structure and radiation angle characteristics, it can be said that it is a light source suitable for Koehler illumination. That is, critical illumination which sets the chip and an illumination object in a conjugate relationship is not suitable for an illumination apparatus since an image of the chip structure leads to lack of illuminant uniformity. On the contrary, in a Koehler illumination optical system that an angle intensity distribution of the light source becomes a position intensity distribution in an illumination area, since there is obtained an intensity distribution which is relatively flat with respect to an angle, a smooth intensity distribution can be readily acquired in an illumination area. A problem is a condensing efficiency of diffused light rays with a wide angle.
As an illumination apparatus which has such an LED as a light source and is used in an image projection apparatus, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-32278 proposes an illumination apparatus which converts an outgoing light ray from an array of LEDs into a substantially parallel light ray by a condensing optical system corresponding to each LED, and converts it to a light flux diameter according to a size of a light modulation element by an afocal optical system constituted by a combination of a convex lens and a concave lens.
Additionally, U.S. Pat. No. 6,227,669 B1 proposes as an illumination apparatus of an image projection apparatus utilizing a plurality of luminous bodies as a light source, a Koehler illumination system which takes out output light rays from a plurality of LEDs by a radiation distribution lens array as a condensing optical system corresponding to each LED and superimposes the light rays from a plurality of the LEDs on a light modulation element such as an LCD by a superimposition lens.
Further, U.S. Pat. No. 6,318,863 proposes as an illumination apparatus used in an image projection apparatus utilizing LEDs as a light source, a critical illumination apparatus which converts light rays with a large angle emitted from a plurality of LEDs into light rays with a small angle by a taper rod corresponding to each LED, and then superimposes them on a light modulation element by a pair of lens arrays corresponding to each taper rod and a superimposition lens. Alternatively, this publication also proposes an illumination apparatus which converts light rays with a large angle emitted from a plurality of LEDs into light rays with a small angle by a taper rod corresponding to each LED and then evenly superimposes them on a light modulation element by a single taper rod.
Furthermore, as a fundamental principle in an optical system, a light preservation rule that a product of an area and a solid angle of a light flux is fixed before and after an optical system is well known. Generally, this product of the area and the solid angle is referred to as Etendue, and this rule is called an Etendue constant rule or a Lagrange constant rule that a product of an object (image) height and a light beam angle (NA) is constant on one plane. That is, in two-dimensional FIG. 1A, assuming that S1 is an area before an optical system 9, Ω1 is a solid angle of its light flux, S2 is an area after the optical system 9, and Ω2 is a solid angle of its light flux, the following expression can be obtained:S1×Ω1=S2×Ω2  (1)Furthermore, in one-dimensional FIG. 1B, assuming that Y1 is an object (image) height before the optical system 9, NA1 is its light beam angle (NA), Y2 is an object (image) height after the optical system 9, and NA2 is its light beam angle (NA), the following expression can be achieved:Y1×NA1=Y2×NA2  (2)